The invention concerns a heat exchanger for cooling cracked gas.
cracked gas, which is generated by thermally cracking hydrocarbons accompanied by the addition of steam, is a mixture of hydrocarbons that differ in molecular weight and partial pressure. To stabilize its molecular composition the gas must be cooled very rapidly from approximately 800.degree.-900.degree. C. to 600.degree.-650.degree. C. The gas is cooled by transferring its heat indirectly to the steam, which acts as a head-absorbing fluid. To maintain the high cooling rate the gas must flow through the heat-exchange tube very rapidly. Atlhough the gas is further cooled from 600.degree.-650.degree. C. depending on the material, the sole purpose of the procedure is to recover heat, and it has little effect on the quality of the gas. This secondary cooling can occur at lower flow rates.
In addition to sufficiently rapid cooling, the gas-end pressure in the tubes in the gas furnace and gas cooler also affects the quality of the resulting gas. A slight loss of pressure in the cooler for example results in lower pressure in the furnace, which increases the yield of ethylene. An attempt is accordingly made in practice to optimize between the rate of flow and the loss of pressure in the flowing gas.
Nested-tube heat exchangers that cool the gas from 800.degree. C. to 400.degree. C. in one draft are employed as cracked-gas coolers. A correspondingly low rate of flow is maintained in the heat-exchanger tubes. Although a heat exchanger of this type is simple in design, the rate of cooling is sometimes too low, especially at the intake, to stabilize the gas at the desired composition.
Two-stage heat-exchanger systems are also known. They are usually single-pipe coolers and cool the gas at a higher flow rate from 800.degree. C. to 500.degree. C. Downstream of these coolers is a separate heat exchanger wherein the gas is cooled to 400.degree. C. at a lower flow rate. Systems of this type cost considerably more.
Finally, the tendency of a heat-exchange tube to become dirty, a tendency that is related to pressure and temperature, must also be taken into consideration. Such contamination occurs when the temperature of individual gas constituents drops below condensation temperature, which depends on partial pressure, and they precipitate on the inner surface of the tube. The result is what is called a coke bed, which increases flow resistance and hence pressure. The temperature of the gas at the exit end increases and less steam is generated. After a certain number of hours of operation, accordingly, the cooler must be stopped to remove the coke bed.
Maintaining the temperature of the tube wall at or above the condensation temperature of the gas constituents in order to decelerate the formation of a coke bed is known. This can be done for example by a two-stage cooling wherein evaporating water is employed as a heat-exchange fluid at the intake and steam at the outlet or in a separate device (German Patent 3 643 801). Decreasing the cooling at the outlet from a cracked-gas cooler by jacketing the outlet end of the heat-exchange tube and allowing a limited amount of evaporating water to flow through the jacket is also known (German Patent 3 715 713).